1. Field of the Invention
The present invention relates to an automatic focusing circuit and more particularly, to an improvement of an automatic focusing circuit for automatically matching the focus relative to an object in response to a video signal obtained from an image sensor, in an image sensing apparatus such as a video camera and an electronic still camera having an automatic focusing mechanism.
2. Description of the Prior Art
Conventionally, in an automatic focusing apparatus used in an image sensing apparatus such as a video camera and an electronic still camera, an approach has been developed for utilizing a video signal itself obtained from an image sensor for evaluating the state in which the focus is to be controlled has According to such an approach, many good characteristics can be obtained. For example, there exists substantially no parallax. In addition, even if the depth of field is small and an object is located in the distance, the focus can be exactly matched. Furthermore, according to this approach, a specific sensor for automatic focusing need not be separately provided, so that the apparatus is very simple as a mechanism.
As an example of such a focus control method utilizing a video signal, a control method referred to as a so-called hill-climbing servo system has been conventionally known. The hill-climbing servo system is described in, for example, U.S. Pat. No. 4,638,364, U.S. Pat. No. 4,614,975, Japanese Patent Laying-Open Gazettes No. 58505/1983 and No. 103776/1985. Briefly stated, a high frequency component of a video signal obtained from an image sensor is detected every one field as a focus evaluating value, the detected focus evaluating value is always compared with a focus evaluating value detected one field before, and the position of a focusing lens continues to be slightly vibrated so that the focus evaluating value always takes the maximal value.
In the above described hill-climbing servo system, if only the slope of a focus evaluating value is detected, the focusing lens is not stopped in the defocused position by driving the focusing lens in the direction of always increasing the focus evaluating value even if the object is changed, so that very good follow-up characteristics can be achieved.
However, such a hill-climbing servo system suffers from the following significant disadvantages caused by continuing to vibrate the position of the focusing lens.
A first disadvantage is that since the focusing lens is not stopped even in the in-focus state, a picture continues to be vibrated even if an object at rest is in focus. For example, the focal length of a lens currently used in a television camera is changed by rotating a focusing ring, so that the angle of field of a recorded image is changed. Therefore, in the above described system in which the focusing ring continues to be vibrated even in the in-focus state, an object on the picture becomes large or small with a particular cycle, resulting in a very unclear picture.
A second disadvantage is directed to a power consumption. There are many cases where a home video camera currently utilizes a battery as a power supply due to the portability thereof. When a focusing motor is always driven as in the above described hill-climbing servo system so that the forward rotation and the reverse rotation are repeated, more power is consumed, as compared with when the focusing motor is rotated in a constant direction, due to in-rush current, so that the time period during which an image can be recorded by using such a battery becomes short.
Additionally, since the focusing ring is always rotated, a problem of wear of a gear occurs, for example.
An automatic focusing circuit to solve these problems is proposed by one of the inventors of the present invention, which is disclosed in Japanese Patent Application No. 273212 filed Nov. 17, 1986.
FIG. 1 is a schematic block diagram showing the proposed automatic focusing circuit, and FIG. 2 is a block diagram showing the details of a focus evaluating value generating circuit shown in FIG. 1.
Referring to FIG. 1, a video camera comprises a focusing ring 2 for moving a focusing lens 1, a focusing motor 3 for driving the focusing ring 2, and an image sensing circuit 4 including an image sensor (not shown) such as a CCD (Charge Coupled Device). The focusing lens 1 may be moved by a piezoelectric element instead of a motor. In addition, the image sensor (not shown) itself such as the CCD instead of the focusing lens may be moved by the piezoelectric element.
An image formed on a surface of the image sensor by the focusing lens 1 is converted into a video signal by the image sensing circuit 4 and inputted to the focus evaluating value generating circuit 5. Referring to FIG. 2 showing the details of the focus evaluating value generating circuit 5, a luminance signal component in a video signal outputted from the image sensing circuit 4 is applied to a synchronizing separator circuit 5a and a gate circuit 5c. The synchronizing separator circuit 5a separates a vertical synchronizing signal VD and a horizontal synchronizing signal HD from the inputted luminance signal and applies the same to a gate control circuit 5b. The gate control circuit 5b sets a rectangular sampling area in a central portion of a picture in response to the inputted vertical synchronizing signal VD and horizontal synchronizing signal HD and a fixed output of an oscillator (not shown). The gate control circuit 5b applies a signal for opening or closing a gate every field to the gate circuit 5c so that passage of the luminance signal is permitted only in the range of the sampling area. The gate circuit 5c may be provided anywhere in the former stage of an integration circuit 5f as described below.
Only the luminance signal corresponding to the range of the sampling area is applied to a high-pass filter 5d every field by the gate circuit 5c. The high frequency component of the video signal separated by the high-pass filter 5d is amplitude-detected by a detector 5e, the detected output being applied to the integration circuit 5f. The integration circuit 5f integrates every field the detected output applied thereto, the integrated output being applied to an A/D converter 5g. The A/D converter 5g converts the integrated value inputted thereto into a digital value and supplies the digital value as a focus evaluating value in the current field.
Description is now made on operation to occur immediately after automatic focusing control is started. Immediately after an automatic focusing operation is started, a focus evaluating value of the first one field outputted from the focus evaluating value generating circuit 5 is first applied to a memory 6 holding the maximum value and a memory 7 holding the initial value and is held therein. Thereafter, a focusing motor control circuit 10 rotates the focusing motor 3 in a predetermined direction. Thereafter, a comparator 9 compares the initial focus evaluating value held in the initial value memory 7 with the current focus evaluating value outputted from the focus evaluating value generating circuit 5 and generates a comparison signal. Accordingly, the focusing motor control circuit 10 initializes the rotational direction of the focusing motor 3 in response to the comparison signal.
More specifically, the focusing motor control circuit 10 rotates the focusing motor 3 in the above described predetermined direction until the comparator 9 generates a comparison output indicating "large" or "small". If and when a comparison output indicating that the current focus evaluating value is larger than the initial focus evaluating value held in the initial value memory 7 is outputted from the comparator 9, the focusing motor control circuit 10 maintains the above described predetermined rotational direction. On the other hand, when a comparison output indicating that the current focus evaluating value is smaller than the initial focus evaluating value is obtained, the focusing motor control circuit 10 reverses the rotational direction of the focusing motor 3.
In the above described manner, initialization of the rotational direction of the focusing motor 3 is completed. Thereafter, the focusing motor control circuit 10 monitors an output of a comparator 8. In order to prevent a malfunction due to noise of the focus evaluating value, the comparator 9 may be adapted not to generate the comparison output indicating "large" or "small" while the difference between the initial focus evaluating value and the current focus evaluating value does not exceed a predetermined threshold value.
On the other hand, the comparator 8 compares the maximum focus evaluating value so far held in the maximum value memory 6 with the current focus evaluating value outputted from the focus evaluating value generating circuit 5, and outputs two kinds of comparison signals (S1, S2), that is, signals in first and second modes in which the current focus evaluating value is larger than or is less than a predetermined threshold value, as compared with the focus evaluating value held in the maximum value memory 6, respectively. If and when the current focus evaluating value is larger than the contents of the maximum value memory 6, the content of the maximum value memory 6 is updated in response to the output S1 of the comparator 8, so that the maximum value of the focus evaluating value so far is always held in the maximum value memory 6.
A signal indicating the position of a focusing ring is generated from the focusing ring 2 corresponding to the position of the focusing ring 2 supporting the focusing lens 1 and applied to a memory 13 holding the position of a focusing ring. The focusing ring position memory 13 is updated in response to the output of the comparator 8 so that the focusing ring position signal in which the focus evaluating value becomes the maximum is always held.
The focusing motor control circuit 10 monitors the output of the comparator 8 while rotating the focusing motor 3 in the direction initialized in response to the output of the comparator 9 as described above. When the output S2 in the second mode in which the current focus evaluating value is decreased to be below the above described threshold value, as compared with the maximum focus evaluating value, is obtained from the comparator 8, the focusing motor control circuit 10 reverses the rotational direction of the focusing motor 3. It is to prevent a malfunction due to noise of the focus evaluating value that the focusing motor is not reversed until the current focus evaluating value is decreased to be below a predetermined threshold value.
After the focusing motor 3 is reversed, a comparator 14 compares the contents of the focusing ring position memory 13 corresponding to the maximum value of the focus evaluating value with the current focusing ring position signal generated from the focusing ring 2. When both coincide with each other, that is, the focusing ring 2 is returned to the position where the focus evaluating value is the maximum, the focusing motor control circuit 10 stops rotation of the focusing motor 3. At the same time, the focusing motor control circuit 10 outputs a lens stop signal LS. In the above described manner, a series of automatic focusing operations are completed.
A memory 11 and a comparator 12 are circuits for resuming an automatic focusing operation by the focusing motor control circuit 10 if the focus evaluating value is changed by more than a predetermined threshold value while the focusing lens is stopped. More specifically, a focus evaluating value at the time point when the automatic focusing operation by the focusing motor control circuit 10 is completed and the lens stop signal LS is generated is held in the memory 11. The comparator 12 compares the contents of the memory 11 with the current focus evaluating value outputted from the focus evaluating value generating circuit 5. If the difference therebetween exceeds the predetermined threshold value, it is considered that an object changed, so that a signal indicating that an object changed is applied to the focusing motor control circuit 10. As a result, the automatic focusing operation by the focusing motor control circuit 10 is resumed, so that an automatic focusing operation following the change of the object can be achieved.
FIGS. 3A, 3B, 4A, 4B, 5A and 5B are graphs showing the relation between the position of the focusing lens or the distance between the lens and the object and the focus evaluating value, respectively. In each of the drawings, an axis of abscissa represents the position of the focusing lens or the distance between the lens and the object, and an axis of ordinate represents the focus evaluating value. As can be seen from the above described drawings, the graph of the focus evaluating value generally is a mountain-like curve. The curve can be made gentle as shown in FIG. 3A or steep as shown in FIG. 3B by changing a constant of a circuit for generating the focus evaluating value, for example, the cut-off frequency of the high-pass filter 5d shown in FIG. 2. The curve of the focus evaluating value directly affects significantly the automatic focusing operation as described below. More specifically,
(1) if the curve is gentle as shown in FIG. 3A, a focus evaluating value of a certain degree of magnitude is obtained even if the lens is significantly displaced from the in-focus position A and an object is defocused, so that reliable hill-climbing control can be achieved.
On the other hand, if the distance between the lens and the object is changed when a particular object is in focus as shown in FIG. 4A, the focus evaluating value is decreased as represented by an arrow in FIG. 4A. However, the decrease is relatively slight. Thus, the decrease cannot be distinguished from the decrease (represented an arrow in FIG. 5A) caused by the change of brightness and lateral movement of the object as shown in FIG. 5A, so that there occurs a problem in continuous automatic focusing operations. More specifically, in the case shown in FIG. 4A, the automatic focusing operation must be resumed. In the case shown in FIG. 5A, the automatic focusing operation need not be resumed. However, here, if only the focus evaluating value is monitored, then both of the above described cases cannot be distinguished. Thus, it is assumed that the automatic focusing operation is resumed if the focus evaluating value is decreased to such a degree. When the state as shown in FIG. 5A occurs, the automatic focusing operation is resumed although it need not be resumed, so that a picture fluctuates. On the other hand, it is assumed that the automatic focusing operation is not resumed if the focus evaluating value is decreased to such a degree. When the state shown in FIG. 4A occurs, the automatic focusing operation is not resumed although it must be resumed, so that a picture remains blurred.
(2) on the other hand, if the curve is steep as shown in FIG. 3B, the above described malfunction does not occur. More specifically, the decrease (represented by an arrow in FIG. 4B) of the focus evaluating value shown if the distance between the lens and the object is changed when a particular object is in focus as shown in FIG. 4B is sufficiently larger than the decrease (represented by an arrow in FIG. 5B) of the focus evaluating value caused by the change of brightness and lateral movement of the object as shown in FIG. 5B. Thus, the above described both cases can be distinguished by observing the focus evaluating value, so that the automatic focusing operation can be resumed as required.
However, on the other hand, since the slope of the focus evaluating value is very small in a base portion of the mountain-like curve, the direction to move the lens cannot be determined if an object is significantly defocused. As a result, the lens continues to move in the reverse direction. In addition, if the lens moves in the correct direction, then the direction of the slope of the curve of the focus evaluating value many change due to noise and slight change of the focus evaluating value, so that the lens begins to move in the reverse direction. Consequently, the automatic focusing operation becomes extremely unstable.
In an automatic focusing camera of a conventional hill-climbing servo system, the focus evaluating value of an intermediate curve between the curve of FIG. 3A and the curve of FIG. 3B is used, so as to minimize these problems. However, the shape of the curve of the focus evaluating value is changed depending on the nature and brightness of the object and the focal length of the lens, so that one of the above described problems (1) and (2) is revealed according to circumstances, whereby a malfunction may occur.
Japanese Utility Model Laying-Open Gazette No. 72967/1986 discloses an automatic focusing circuit in which an output of a first filter having a high first cut-off frequency (for example, 600 KHz) and an output of a second filter having a low second cut-off frequency (100 KHz) both receiving a video signal corresponding to a set sampling area are selectively used to generate a signal for hill-climbing servo control. More specifically, until a signal obtained by detecting the output of the first filter attains a predetermined level, an automatic focusing operation is performed using a signal obtained by detecting the output of the second filter and then, using a signal obtained by detecting the output of the first filter. According to such an automatic focusing circuit, the above described unstable automatic focusing operation can be prevented to a certain degree.
However, in such an automatic focusing circuit, since the level is determined based on a detected output which is an analogue signal, the problem occurs as described below. FIGS. 6A and 6B are diagrams showing diagrammatically a first detected output corresponding to a sampling area in one field, respectively. In each of the drawings, an axis of abscissa represents the time, an axis of ordinate represents the magnitude of an analogue signal obtained by detecting the output of the first filter in one field, and a dotted line represents a threshold value. As shown in FIG. 6A, when the first detected output exceeds the threshold value, correct automatic focusing control is performed by the first detected output. As shown in FIG. 6B, when the first detected output does not reach the threshold value, an automatic focusing operation is performed by a second detected output. However, in FIGS. 6A and 6B, there is no large difference in a high frequency component included in a video signal. Thus, if the level is determined based on such a detected output as it is, a stable automatic focusing operation cannot be performed.